How to make permanent custom tab and removing some submenu in tab tools (T)?
What you are referring to as a submenu is the header of a panel. The template that you created includes a class that inherits from a Panel, the bl_category value in the panel class decides what tab the panel will be shown in.

Tabs only exist if there is a panel to be shown in them. When there are no panels to show the tab is removed. To make a tab permanent you need to have a panel to show in it that is always available, this may be a python script set to auto-run on startup that is saved to your startup.blend or an addon that is set to be enabled on startup, you do that by enabling the addon and then saving user settings.